Coming from a 3.0 z4, I've got absolutely no issue with the performance of the 630i.
If anything it feels quicker than the z4, I know on paper it isn't, but it just whooses forward, I think higher redline and longer gears help with this illusion .
As for reliability issues, I think most are pitched at the v8 versions I.e oil leaks etc.
I think that the 6 cylinder versions are more tried and tested.
What went wrong with yours to cost £1700?